The MinIO cache key only encoded page number, so all filter combinations
hit the same cache entry and returned unfiltered results. Introduce
BrowseFilteredHTMLKey() that encodes sort/genre/status into the key
(e.g. novelfire.net/html/new-isekai-completed/page-1.html); falls back
to the original page-only key for default filters to preserve existing
cached pages.
When a book is not in the local DB and the preview endpoint is hit,
metadata and the chapter index are now saved to PocketBase in the
background. Subsequent visits load from the local store instead of
scraping live. Chapter text is not fetched until an explicit scrape job
is triggered.
